1. Field of the Invention
This invention relates to a method and apparatus for determining the underground stress state and material properties and, more particularly, to determining these values without dependence upon the unrealistic assumption that the underground media is an ideally elastic and homogeneous medium. This new method and apparatus may be designed to repeat the measurement automatically at a same site as needed as a function of time and aging of the ground.
2. Description of Related Art
U.S. Pat. No. 5,576,485 documents the original introduction of the Single Fracture Method for measuring both the stress state and the material properties in underground media. This groundbreaking invention is based on using an expandable borehole probe to fracture the borehole wall along a single predetermined plane that extends through the borehole axis. The probe is expanded by high pressure hydraulic fluid and the diametrical expansion is monitored by high accuracy sensors. A stable force balance that is established between the ambient ground stress vector normal to the defined fracture plane and the pressure that initiates and reopens the single-fracture plane reveals the tensile strength of the underground media, as well as the magnitude of the ambient stress that is normal to the fracture plane. By utilizing a multiple number of stress vectors obtained at a position, stress field tensor can be automatically calculated by the data analysis software installed within the stressmeter. The tensor is given in terms of maximum stress, minimum stress and their angular orientation.
The original single-fracture method was based on the assumption of an ideally elastic and homogeneous condition of underground rock media. This theoretical assumption caused a significant amount of error because of the fact that the natural rock media are found to be highly inhomogeneous with different elastic coefficient value found in different orientations. This inhomogeneity problem has been well demonstrated by irregularity of all the conventional methods of stress measurement, which are based on the assumption of ideally homogeneous elastic ground. This fundamental problem has been resolved by the advanced single-fracture method developed for general inhomogeneous ground based on the force balance principle of the present invention.
Extended experience with practical applications of the single-fracture method revealed some aspects that require improvement. For example, the loading surface of the probe did not achieve the required maximum saturated frictional engagement with the borehole wall to create an accurate single fracture reopening along the predefined plane. The lack of the required friction force diminishes the accuracy and usefulness of the method.
It was also observed that the end seal structures of the introductory designs were subject to failure in rough ground, in which features such as voids or pre-existing fractures would allow the end seal to expand excessively and fail. The original methodology did not recognize this important requirement.
The most important improvement made by the advanced method of the present invention is its expansion of the areas of application to include complex ground including elastic, non-elastic and inhomogeneous grounds successfully. It overcomes the basic difficulty of the original method, which is limited only to an ideally elastic condition. Ideally elastic and homogeneous ground is not common in ordinary ground.